Hydrogen, carbon monoxide and/or synthesis gas are produced by the catalytic reaction of hydrocarbons and steam at high temperature and elevated pressure. In a conventional steam-methane reformer, the heat required for the endothermic reforming reaction is provided by the combustion of fuel in a reformer furnace. In an autothermal reformer, the heat duty is provided by partial combustion of the process feed gas prior to reforming in the catalyst bed. The reformer size is dependent upon the heat duty requirements of the reforming reaction. The cost of the reforming can be as much as 40% of the equivalent costs for the synthesis gas plant. The reformer fuel for a conventional steam-methane reformer accounts for 10 to 30% of the plant energy costs. Costs are similar for the oxygen and the portion of the process feed gas which is combusted in an autothermal reformer.
A convective heat transfer reformer can be integrated with either a conventional steam methane reformer or an autothermal reformer to utilize high level waste heat to reform about 20-40% of the feed gas. A convective heat transfer reformer can recover waste heat from the synthesis gas produced in a conventional or autothermal reformer to heat and reform feed gas. By reforming a portion of the steam-methane feed in a convective heat transfer reformer, the size of the primary reformer, either conventional or autothermal, is reduced. Conventional reformer fuel requirements or autothermal reformer oxygen and feed requirements are also reduced.
A convective heat transfer reformer can consist of a pressurized shell and a bundle of tubes containing conventional reforming catalyst. Feed gas consisting of steam and methane flows through the catalyst filled tubes in a direction countercurrent to hot synthesis gas flowing on the shell side. The feed gas is heated and reformed as it flows through the catalyst filled tubes.
Several processes have been proposed in the prior art, that incorporated a conventional steam-methane reformer with either a convective heat transfer reformer, or a prereformer using high nickel content prereforming catalyst. Several prereforming catalysts have also been proposed.
U.S. Pat. No. 4,824,658 proposes a convective reformer used to partially reform a feed of steam and hydrocarbons. The partially reformed feed is further reformed in a primary steam reformer furnace or autothermal reformer. Waste heat recovered from the primary reformer effluent supplies the heated reaction for the partial reformation of the feed in the convective heat transfer reformer.
U.S. Pat. No. 4,919,844 proposes a process that integrates a convective heat transfer reformer with a conventional steam-methane reformer. A portion of the hydrocarbon-steam feed stream is reformed in the convective heat transfer reformer. The heat of the reaction for the convective heat transfer reformer is supplied by recovering waste heat from the reformed reaction products of both the conventional reformer and the convective heat transfer reformer.
U.S. Pat. No. 4,631,182 offers a two-step process for the production of hydrogen and/or carbon monoxide. In the first step, a portion of the hydrocarbon and steam feed is passed through an adiabatic reactor containing a prereforming catalyst. The feed is reformed at low temperature and normal operating pressure. The effluent is combined with the outlet from a direct reduction iron ore furnace. In the second step, the combined stream is further reformed in a tubular furnace reformer to produce hydrogen and carbon monoxide for use in the reduction furnace. This process shifts a portion of the reforming duty to an adiabatic packed bed reactor and reduces the reformer fuel requirements.
U.S. Pat. No. 4,104,201 describes a steam reforming catalyst suited for low temperature and high pressure operation. The catalyst contains 25 to 75% nickel and some ruthenium. Resistance to polymer deactivation is asserted.
U.S. Pat. No. 4,417,905 proposes a 50 to 65% nickel catalyst and process for the production of methane-containing gases from hydrocarbons and steam. The new catalyst has a higher nickel concentration and operates at lower temperatures than conventional reforming catalyst. The catalyst is also resistant to sintering and deactivation by polymer formation.
U.S. Pat. No. 3,988,425 couples an adiabatic prereformer with a tubular reformer for reforming a light hydrocarbon feed stock having an average carbon number not greater than 15, at a steam to carbon molar ratio between 1.1 and 1.7. The prereformer catalyst contains 25 to 70 wt.% nickel, operates at temperatures of 300.degree. to 500.degree. C. and cracks the light hydrocarbon feedstock to methane. The methane-steam mixture is then sent to the tubular reformer in which hydrogen and carbon monoxide are produced.
U.S. Pat. No. 4,810,472 discloses an apparatus for conducting an endothermic catalytic reaction wherein closed ended, externally heated reformer tubes are employed. The inner return tubes are insulated so that there is only a small temperature drop between the reacted gas leaving the catalyst zone and entering the return tubes and the gas leaving the return tubes. The outer, closed end, tubes preferably have fins to increase the surface area, and are surrounded by sheaths through which a heating medium passes. The heating medium is preferably hot gas obtained by subjecting the primary reformed gas to secondary reforming.
The literature article, "Opportunities for Savings with Prereformers" by D. N. Clark and W. G. S. Henson delivered at the 1987 Ammonia Symposium, American Institute of Chemical Engineers, Aug. 16, 1987, discloses the integration of conventional reformers with adiabatic prereformers. The figures in the article, particularly FIG. 3B and FIG. 3C, disclose an adiabatic prereformer operating using the waste heat from the flue gas of a conventionally fired primary reformer to reheat the prereformer effluent.
The integration of the convective heat transfer reformer with a primary steam methane reformer in a hydrogen or synthesis gas plant, reduces the size and costs of the primary reformer and energy requirements of the plant. The costs savings associated with the reduced primary reformer size can be offset by the cost of the convective heat transfer reformer however. To achieve the full benefit of an integrated primary reformer and convective heat transfer reformer process cycle, capital costs must be further reduced. The present invention achieves desirable capital cost reductions by the unique combination of features in a convective heat transfer reformation as set forth below.